1. Field of the Invention
The present invention relates to a motor control circuit for a VCR (videocassette recorder).
2. Description of the Prior Art
In VCRs, a drum motor and a capstan motor are usually controlled by separate control systems. Accordingly, if the feed speed of a magnetic tape, which is fed by a capstan rotated by the capstan motor, and the scanning speed of a magnetic head, which is attached on a drum rotated by the drum motor, deviate from predetermined speeds relative to each other, the time intervals between horizontal synchronizing signals included in a reproduced video signal fluctuate. That is, the horizontal frequency f.sub.H fluctuates. In order to compensate for such fluctuation of f.sub.H, it is necessary to compensate the speeds of the drum motor and the capstan motor relative to each other. And, the f.sub.H compensation, as required in special playback modes such as search modes, usually assumes constant speeds in the motors. While the motors are changing their speeds, the f.sub.H compensation is corrected differently for the drum control system and the capstan control system in accordance with their different responses. For this reason, the amount of f.sub.H compensation during a transient change of the search speed is often different between the two control systems, causing color misplacement in images reproduced during the transient speed change.
For example, when the speed is switched from a high-speed search A to a low-speed search B, the drum-side frequency changes rapidly from the frequency for search A to that for search B, as shown by the curve (a) in FIG. 1, whereas the capstan-side frequency changes slowly, as shown by the curve (b). This difference between the change rates is the cause of color misplacement that occurs during a speed change.
In a conventional circuit, this problem is solved by applying forced braking to the capstan control system to adjust the capstan-side change characteristic (a) to the drum-side change characteristic (b). Specifically, as shown in FIG. 2, in an early period T of a speed change, forced braking is applied to the capstan motor to make it decelerate rapidly so that the capstan-side change characteristic (b) is adjusted to the drum-side change characteristic (a). Here, the forced braking is realized by using a `heavier` filter (that is, a filter with a larger time constant) provided somewhere in the path of the speed control signal.
However, the conventional circuit is defective in that, since the rate of the speed change (thus the response of the speed change) depends on how much braking is applied, it not only necessitates much cut-and-try effort on the designer's part but also often causes characteristics variations among circuits produced.